Spitzer Reveals Unexpected Disks Around Interacting Stars (Forwarded)

Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Tucson, Arizona

For More Information:

Douglas Isbell
Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Phone: 520/318-8230

Whitney Clavin
Jet Propulsion Laboratory
Phone: (818) 354-4673

EMBARGOED FOR RELEASE: 9:20 a.m. EST, Tuesday, January 10, 2006

RELEASE NO: NOAO 06-04

Spitzer Reveals Unexpected Disks Around Interacting Stars

New Spitzer Space Telescope observations of an unusual class of
interacting binary stars detected excess amounts of infrared radiation,
suggesting that these odd objects are surrounded by large disks of cool
dust.

The results reported today in Washington, DC, at the 207th meeting of
the American Astronomical Society (AAS) were produced by one of six
teams of professional astronomers and high school teachers participating
in a unique program co-sponsored by the Spitzer Science Center and the
National Optical Astronomy Observatory (NOAO).

The type of cataclysmic variable system being studied by the team
consists of a highly magnetic white dwarf star (a "dead" remnant star
formed from the core of a star like our Sun when it exhausts the
available fuel to support nuclear fusion) and a very low mass, cool
object similar to a brown dwarf. The two objects orbit so closely --
about the distance from Earth to the Moon -- that they make a complete
revolution about each other in only 80-90 minutes. The white dwarf is
Earth-sized but weighs about 60 percent of the mass of the Sun, while
the companion star is Jupiter-sized but has about 40-50 times the mass
of Jupiter.

The high mass of the white dwarf and the closeness of the companion
result in mass exchange between the two stars. The gravitational
influence of the white dwarf squeezes the companion star into a teardrop
shape, and matter squirts from its pointed end toward the white dwarf,
like water from the nozzle of a garden hose. This material eventually
falls onto the white dwarf, causing tremendous heating of its atmosphere
and the emission of a large amount of energy from X-rays to the far
infrared.

A team of astronomers and teachers led by Steve B. Howell of NOAO
observed four of these types of binaries with NASA's Spitzer Space
Telescope in an attempt to study the cool, low-mass object in the pair:
EF Eridanus, V347 Pav, GG Leo and RX J0154.

To their surprise, excess infrared emission was discovered around all
four. The team's current best model for its origin is a large, cool
circumbinary dust disk with a temperature of about 800-1,200 Kelvin
(980-1,700 degrees Fahrenheit).

"Our explanation at this point is that the emission originates from a
large, relatively cool disk of dust encircling the entire binary
system," Howell says. "The discovery of dust disks around these old
interacting binaries is very exciting. We have shown our initial results
to a variety of specialists, and nobody yet has a better idea of what we
are seeing."

Such circumbinary disks have been predicted on theoretical grounds and a
few observational studies have attempted to find them, with mixed
results. The disks may be the remains of the large "mass-loss" episode
that occurred during the formation of the white dwarf. They also could
be composed of material spewed from the binary in the form of strong
winds (like a very dense version of our Sun's solar wind), or material
that was ejected during one or more previous nova explosions. Cyclotron
emission due to the large magnetic field of the white dwarfs in these
particular binaries cannot be eliminated completely as another potential
source of at least part of the infrared emission.

"A number of ideas are on the table, as well the possibility of some
still-unknown process," Howell adds. "These objects are ripe for further
study."

Only two other white dwarfs (including one newly discovered) are known
to be encircled by a dust disk -- stars named G29-38 and GD362. Unlike
the cataclysmic variables studied by Howell's team, both of these are
single white dwarfs, and the source of their dust disks is not known for
certain. Dust disks made up of "left over" material from the star
formation process are known to exist around very young stars and have
been discovered around Sun-like stars as well. Some of these latter
disks are known to harbor planetary-type objects, orbiting in cleared
out "rings" within the disk.

"While we have no evidence for planetary objects in our disks, the
possibility does exist," Howell adds. "More work must be done to prove
the infrared excess is from a disk and, if true, to discover its
properties such as density and composition. We also would like to see if
these disks exist in every interacting binary of this type or only in
some. Their presence would greatly change our concept of the evolution
of such systems."

These types of systems are important because they give astronomers
insight into the accretion, or "mass transfer," process that also plays
a role in the formation of stars and planets, according to team member
Donald W. Hoard, an astronomer at the Spitzer Science Center in
Pasadena, California.

"Cataclysmic variable accretion is one of the least complicated forms of
mass transfer in the Universe," Hoard says. "These systems are great to
observe, because unlike accretion during the formation of stars and
planets, or around supermassive black holes in far off galaxies, the
process in cataclysmic variables happens on relatively short, human
timescales."

A color graphic to illustrate this result is available at
http://www.noao.edu/outreach/press/p...04.html#images

Other members of the research team reporting in poster 70.17 today at
the AAS meeting include Carolyn Brinkworth of the Spitzer Science
Center, and physics teachers Howard Chun from Cranston High School in
East Cranston, Rhode Island; Beth Thomas of Great Falls Public Schools
in Great Falls, Montana; and, Linda Stefaniak of Allentown High School,
Allentown, New Jersey.

Chun, Thomas and Stefaniak are graduates of NOAO's Teacher Leaders in
Research Based Science Education (TLRBSE)
[http://www.noao.edu/outreach/tlrbse/], a teacher professional
development program funded by the National Science Foundation. Twelve
TLRBSE teachers were competitively selected in the fall of 2004 to work
in six teams that were awarded three hours of Director's discretionary
observing time with Spitzer.

"This opportunity has allowed my students and myself to participate in
authentic astronomy research," Thomas says. "It has given me a deeper
understanding of the world of infrared and astronomy, while reinforcing
how we teach the process of problem solving and how answers are sought
in the science community. The experience has been very enlightening,
extremely rewarding and genuinely stimulating."

Another six TLRBSE teachers were just selected for a second round of the
program, and these teachers met with their astronomer partners during
this AAS meeting to begin planning new research. Background information
on the Spitzer-TLRBSE program and the experience of the teachers and
their students in this group can be found on the Web at the Spitzer Web
Site,
http://www.spitzer.caltech.edu/Media...ings/20050816/

The National Optical Astronomy Observatory is operated by the
Association of Universities for Research in Astronomy Inc. (AURA), under
a cooperative agreement with the National Science Foundation.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope
mission for NASA's Science Mission Directorate. Science operations are
conducted at the Spitzer Science Center at Caltech. JPL is a division of
Caltech.